Drilling systems are frequently used to provide cylindrical holes in metallic workpieces. The cutting or boring action of the drill system may be carried out by an elongated, substantially cylindrical drilling tool, such as a combination of a tool holder and a drill insert, which is selectively attached thereto such as the type commonly referred to as a spade-type drill. Such an arrangement may then be used in an application wherein one end of the tool holder is securely mounted in a driving apparatus, which rotates the holder about its longitudinal axis. At the opposite end of the elongated tool holder, the cutting insert engages the material to be cut. Alternatively, the workpiece may be made to rotate relative to the holder and cutting insert, such as in positioning the holder in the tail stock of a lathe or the like. Further, the tool and workpiece may be made to rotate relative to one another. The use of cutting inserts allows for quick changing of the insert upon wear of the cutting surfaces instead of the entire tool, and allows for one tool to be used for a variety of different boring applications by simply changing the insert and not the entire drill assembly.
One problem with prior art spade-type cutting tools is that insert is typically configured as a flat blade and the holder is configured with a straight flute. Even when used with flush channels through the holder, these types of drill assemblies are best suited for drilling shallow holes at relatively slower speeds due to their poor chip removal. Helical flutes are provided in typical twist drills to help in chip removal. Large helix angled flutes (often 20 degrees or above) are used for effective chip removal at high drilling speeds.
Attempts have been made in the prior art to combine the versatility and cost-effectiveness of the spade-type insert and holder with the benefits of helical flutes, however, these efforts have resulted in complex or inadequate blade retaining systems as well as complex shaped blades which negate the cost benefits of the replaceable blade insert. Other prior art attempts have combined a helical flute with a portion of a straight flute at the drill insert connection end in order to accommodate a flat drill insert. However in terms of chip removal, improved performance can be obtained if the helical flute is adjacent the cutting edges. In this prior art configuration, the chips are first transported by a straight portion resulting in loss of efficiency in removing chips and forcing a slower cutting speed for the tool. In addition, the straight cutting edge of the flat cutting blade does not typically dissipate heat as well as a curved cutting edge. If the point does not adequately conduct heat away from its cutting edges, the temperature buildup will “burn” the point and diminish the life of the drill bit. The heat generated at the lip of the drill point is directly related to the load and stresses the lip is subjected to. The more efficiently load stresses are dissipated, the less heat is built up at the cutting edge of the drill point.
Some prior art inserts have been developed with radially curved cutting edges. However, these prior art inserts are apt to direct chips directly into the clamp arms of the holder during operation of the tool. The clamp arms and the holder are made of a steel material that is significantly softer than the hard carbide material typically used to manufacture the drill insert. The chips impacting against the clamp arms erode and wear the clamp arms. This chip erosion of the clamp arms significantly reduces the life of the holder.
With relation to the present continuation-in-part application, another problem associated with the prior art drill inserts is related to ensuring proper placement and attachment of the insert within the holder. Ideally the centerline of every drill insert will correspond with the axial centerline of the holder, and the cutting surfaces or point geometry formed precisely relative to this centerline. A change in the index position of the new bit within the slot of the holder can cause significant error. Additionally, the insert itself must be machined with cutting surfaces and an overall outside diameter which must be kept to within very close tolerances to avoid other possible errors when assembled with a tool holder. Of significant importance is maintaining the outside diameter also in high tolerance with respect to the axial centerline of the tool holder. In the methods of manufacturing drill inserts of this type, no effective remedy to errors with respect to overall outside diameter and relative positioning of the cutting surfaces associated with the insert have been found. This in turn causes TIR errors when assembled with a holder and used in a drilling operation. There is therefore a need for a drilling tool, wherein a drill insert can be precisely manufactured to mate with a tool holder in a manner which provides self-indexing and self-alignment of the drill insert with respect to the tool holder.
A variety of methods of attaching a drill insert to the holder have been attempted to properly index the drill insert within the holder. Such attempts include using a locating slot formed on the bottom of the drill insert opposite the cutting edge. The slot mates with a corresponding pin or dowel positioned in the holder. The dowel pin is of a nominal size and must be manufactured within a tolerance limit, and the corresponding slot in the drill insert also has a size tolerance to allow the dowel to be inserted. These manufacturing tolerances cause a clearance to occur between the locating dowel in the holder and the locating slot in the drill insert. This clearance does not promote repeatability in placement of the drill inserts within the holder, and instead adds to the index error as described above. While standard spade drills utilize holders with arms that cover the drill insert locating slot, in configurations having central, radially extending bosses and helical flutes, the arms cannot extend across the center of the spade drill, thus leaving the slot at least partially uncovered and susceptible to wear and creating a location for chip build-up.
Other configurations include biasing the insert against one or more seating surfaces, such as by offsetting the retaining screw hole in the tool holder with respect to the corresponding hole in the drill insert. This offset causes a camming action to occur when the retaining screw in tightened forcing the drill insert to be seated against the seating surfaces. Although this may lead to reduced errors in some cases, it is also possible that if the drill insert is positioned in the slot in an orientation which differs from the position of the insert when manufactured, the error could be increased. Though it may be possible to machine the locating slot in the insert to be exactly centered and have very low tolerances on the finished size of the slot, this requires extremely tight tolerances be kept for the placement and dimensions of the locating slot with respect to the centerline of the drill insert. Manufacturing within the tight tolerances required for this indexing system would dramatically increase the price of each drill insert. This would also make use of the insert and assembly holder more difficult, as the insert would have to be precisely positioned to receive the pin, and forced into the proper location about the locating pin.
It therefore would be desirable to have a drill tool assembly which allows a drill insert to be precisely positioned with respect to the tool holder to reduce errors in the positioning of the insert relative to the holder, but which allows simplified and consistent assembly of the insert with the holder. It would also be desirable to provide a method of manufacturing drill inserts for use with a holder which allows the drill insert to be manufactured in a manner to be consistently and precisely positioned with respect to any holder with which it is used, and to minimize errors in such positioning.
Therefore, there remains a need in the art for a drill insert tool that overcomes one or more or the disadvantages identified in the prior art.